The present invention relates to a cathode active material for a high-capacity alkaline storage battery, which mainly consists of a metal oxide containing nickel as a primary metallic element, as well as to a cathode using such an active material.
With the recent advance in semiconductor technology, development of small-sized, light-in weight, multi-functional electronic apparatuses, such as cellular phones and notebook computers, has been proceeded rapidly. The small-sized, light-in weight alkaline storage battery has thus been highly required for the power source of such portable apparatuses.
Nickel oxide (NiOOH) is generally used for the active material of the cathode for an alkaline storage battery. A three-dimensional foamed nickel porous substrate having a high porosity (95%) has replaced the conventional sintered substrate. The electrode obtained by closely packing nickel oxide powder into the foamed nickel porous substrate (foamed metal electrode) has been adopted for industrial applications (U.S. Pat. No. 4,251,603). This remarkably improves the energy density of the nickel cathode.
One important technique for providing a nickel cathode of high energy density improves the method of manufacturing the active material, nickel oxide powder. The conventional method of manufacturing nickel oxide powder makes an alkaline aqueous solution, such as sodium hydroxide, react with an aqueous solution of nickel salt to precipitate nickel hydroxide. After the growth of the crystal by aging the precipitate, the nickel hydroxide crystal is mechanically ground. This method requires the complicated process and gives the nickel hydroxide powder of non-fixed shape. This makes it difficult to provide an electrode of high packing density.
As described in Japanese Examined Patent Publication No. Hei 4-80513, the improved manufacturing method has been proposed, which repeats the process of making ammonia react with an aqueous solution of nickel salt to form a nickel-ammonium complex and the process of making an alkali react with the complex to yield nickel hydroxide, thereby causing nickel hydroxide to be grown. Namely this method makes nickel hydroxide deposit on the existing nickel hydroxide. This method enables continuous production of nickel oxide and reduces the manufacturing cost. The quasi spherical shape of the resulting oxide enables the high-density packing.
The active material of nickel oxide thus obtained is high-density particles grown to have a large particle diameter of several tens .mu.m. This decreases the electronic conductivity of the active material itself and thereby lowers the charge and discharge efficiency of the electrode. Some countermeasures have been proposed; for example, addition of metallic cobalt, cobalt oxide, or metallic nickel to compensate for the electronic conductivity or incorporation of a metallic element other than cobalt or nickel into the active material to form a solid solution and improve the charge and discharge efficiency.
As described in Power Sources 12, p203 (1988), Cd and Co are known examples of the metallic element incorporated into the crystalline nickel oxide to improve the charge and discharge efficiency. Cadmium-free batteries are, however, required from the environmental point of view. Incorporation of Zn and incorporation of three elements, Co, Zn, and Ba, have been proposed, instead of cadmium (U.S. Pat. No. 5,366,831). The technique of incorporating a different metallic element into nickel oxide and forming a solid solution for the purpose of the high charge and discharge efficiency has been known and disclosed, for example, in Japanese Unexamined Patent Publication No. Sho 51-122737.
The improvements in structure of the substrate, particle shape of the active material, composition of the active material, and additives have significantly improved the energy density of the cathode. The practically used cathode has the energy density of about 600 mAh/cc. As described above, however, the requirement for improving the energy density of the power source for small-sized, portable apparatuses has been enhanced more and more. Another approach to improve the energy density of the battery modifies the anode and the cathode, the electrolyte, and the separator as well as its cell structure.
Practical use of a metal hydride having a high energy density for the conventional cadmium anode (Power Sources 12, p393 (1988)) has raised the volume energy density of the anode to at least double the energy density of the cathode. The technical advance, such as formation of a thinner film separator or high-density packing of the electrode material, has remarkably improved the energy density but substantially reached the limit.
The most effective technique for further improving the energy density increases the energy density of the cathode, which occupies almost half the volume of the battery.
There are some approaches of increasing the packing density of the electrode material to improve the energy density of the cathode; for example, an improvement in tap density of the active material particles, reduction of the amount of the additives, and reduction of the amount of the metal included in the foamed nickel substrate. These techniques, however, have substantially reached the limit. It is accordingly necessary to modify the active material itself with a view to improving the reactivity and the reaction order.
The nickel oxide conventionally used as the active material of the cathode has the structure of .beta.-Ni(OH).sub.2 (divalent oxide) when being packed into the electrode substrate. It is said that the .mu.-Ni(OH).sub.2 is reversibly changed to .mu.-NiOOH (trivalent oxide) through the charge-discharge reaction accompanied with an exchange of one electron. The .mu.-NiOOH in the charged state is excessively charged and oxidized to the highly oxidized structure .gamma.-NiOOH (valence: 3.5 to 3.8). The .gamma.-NiOOH is an irreversible stoichiometric material having the disordered crystal structure (J. Power Sources 8, p229 (1982)).
This .gamma.-NiOOH is electrochemically inactive and results in a voltage drop and a decrease in capacity. The wider inter-layer distance of .gamma.-NiOOH expands the volume of the electrode and thereby causes a lot of troubles, for example, the defected contact of the active material with the electrically conductive agent or the substrate, release of the active material from the substrate, and intake of water molecules to dry up the electrolyte. It is accordingly required to interfere with production of .gamma.-NiOOH.
In order to attain the high energy density of the active material including nickel oxide as the base material, it is extremely important to take advantage of the high-order oxide, .gamma.-NiOOH. One proposed material has the structure similar to an .alpha.-type hydroxide obtained by substituting part of Ni with another metallic element, such as Mn(III), Al(III), or Fe(III), and taking anions and water molecules between the layers (U.S. Pat. Nos. 5,348,822 and 5,569,562). It is thought that this oxide is reversibly changed to the high-order oxide having the structure similar to .gamma.-NiOOH through charge and discharge. This oxide, however, has a wide inter-layer distance and a low density (true density), which make the high density packing difficult, and is not practical.
The inventors of the present invention have noted the active material that has the .beta.-type crystalline structure in the process of filling into the electrode and is reversibly changed to the high-order oxide, .gamma.-NiOOH through charge and discharge. The inventors have proposed modification of the nickel oxide by incorporating another metallic element with a view to attaining the charge and discharge reaction accompanied with an exchange of more than one electron. A composition including Mn as the primary component has also been proposed for the metallic element incorporated into the nickel oxide (Japanese Unexamined Patent Publication No. Hei 9-115543). As disclosed in this reference, incorporation of Mn into the nickel oxide enhances the mobility of protons and the electronic conductivity and thereby improves the utilization.
The solid solution nickel oxide with Mn incorporated therein has already been proposed in Japanese Unexamined Patent Publication No. Sho 51-122737, No. Hei 4-179056, and No. Hei 5-41212. The inventors have also noted the solid solution nickel oxide with Mn incorporated therein. The inventors have found that this solid solution nickel oxide is readily charged and oxidized to the .gamma.-phase by regulating the valence of the incorporated Mn and discharged to attain the high-order reaction having the valence of not less than 1.2. The inventors have also proposed the method of synthesizing such a solid solution nickel oxide to attain the high density.
As described above, one proposed method uses the solid solution or eutectic mixture nickel oxide with Mn incorporated therein for the cathode active material, in order to improve the charge and discharge efficiency and the reaction order. In the proposed material, however, .gamma.-NiOOH is produced during normal charging and allows reversible charge and discharge. The expansion and contraction of the active material in the electrode may accordingly destroy the conductive network of the cobalt compound. This interferes with production of the .gamma.-phase and result in a little lower cycle stability, compared with the conventional nickel oxide in which charge and discharge reaction proceeds with an exchange of approximately one electron.
The object of the present invention is thus to attain the remarkably high energy density by effectively utilizing the .gamma.-phase for the charge and discharge reaction and to provide a cathode active material for an alkaline storage battery having the excellent cycle life property.